Bismaleimides per se occupy a prominent position in the spectrum of thermosetting resins. Indeed, several bismaleimides are commercially available. Bismaleimide resins are used as starting materials for the preparation of thermoset polymers possessing a wide range of highly desirable physical properties. Depending on the particular resin and formulation, the resins provide cured products having excellent storage stability, heat resistance, as well as good adhesive, electrical and mechanical properties. Accordingly, bismaleimide resins have been used for the production of moldings, heat-resistant composite materials, high temperature coatings and for the production of adhesive joints. Typically, however, in any particular resin formulation there is a trade-off between the various properties. For example, in the formulation of "snap" cure adhesives (i.e., adhesives that cure in two minutes or less at.ltoreq.200.degree. C.), it is desirable to use a system which does not require the addition of diluent to facilitate handing. In other words, snap cure products require formulations containing 100% reactive materials. Thus, it is desirable to prepare snap cure resins which are liquid at or about room temperature (i.e., low viscosity materials) for ease of handling.
Unfortunately, up until now, it has not proved possible to formulate bismaleimide compositions that are both quick curing, easy to handle (i.e., liquid at or about room temperature), and have low moisture uptake. Consequently, it is a desideratum to provide thermosetting bismaleimide resin compositions that produce cured resins exhibiting a combination of highly desirable physical properties, including a combination of rapid curing and low water absorption.
A particular disadvantage of the use of bismaleimide resins for the types of applications described above is that, at room temperature, such materials exist as solid resins which require the addition of liquid diluents, in order for such resins to achieve a useful and processable viscosity. This difficulty has been compounded by the poor solubility of bismaleimides in organic solvents. This poor solubility generally necessitates the use of polar diluents, such as N-methyl-2-pyrrolidone or dimethylformamide. These diluents are undesirable, inter alia, from the viewpoint of environmental pollution. Therefore, it is another desideratum to provide bismaleimide resins that require little, if any, non-reactive diluent to facilitate handling.
One approach to solving the problem of a need for a diluent has been to use reactive liquid diluents. For example, the co-cure of simple bismaleimides with relatively simple divinyl ethers is known in the art. The use of such diluents is advantageous in that these materials become incorporated into the thermosetting resin composition, and hence do not create disposal problems. However, the range of suitable liquid reactive diluents is very limited. Many of the available diluents are restricted by the low boiling points thereof, and, therefore, the high volatility thereof; by the odor of such materials; by the toxicity of such materials and/or problems with skin irritation induced thereby; by the poor ability of such materials to solubilize bismaleimides; by the high viscosity of such materials, which, again, limits the bismaleimide solubility and also leads to little or no tack in the formulation; by the poor thermal stability and/or hydrolytic stability of such materials; by the incompatibility of such materials with other formulation modifiers, and the like. In particular, since the diluents become an integral component of the thermosetting resin composition, they necessarily influence its properties. Consequently, it is another desideratum to provide combinations of bismaleimide resins with reactive diluents which do not suffer from the above-described drawbacks and that produce cured resins exhibiting a combination of highly desirable physical properties, including rapid curing and low water absorption.
Accordingly, there has existed a definite need for bismaleimide resins that produce cured resins exhibiting a combination of highly desirable physical properties, including rapid curing and low water absorption. There has existed a further need for bismaleimide resins that require the additions of little, if any, non-reactive diluent to facilitate handling. And there has existed a still further need for combinations of bismaleimide resins with reactive diluents which do not suffer from the limitations of known reactive resins and that produce cured resins exhibiting a combination of highly desirable physical properties, including rapid curing and low water absorption. The present invention satisfies these and other needs and provides further related advantages.